1. Field of the Invention
The present invention relates generally to fuel cell systems, and more particularly to a new and improved ink compositions for gas diffusion media components of Proton Exchange Membrane (PEM) fuel cell systems.
2. Discussion of the Related Art
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In PEM-type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements, sometimes referred to as the gas diffusion media components, that: (1) serve as current collectors for the anode and cathode; (2) contain appropriate openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts; (3) remove product water vapor or liquid water from electrode to flow field channels; (4) are thermally conductive for heat rejection; and (5) have mechanical strength. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (e.g., a stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the MEA described earlier, and each such MEA provides its increment of voltage.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
Examples of technology related to PEM and other related types of fuel cell systems can be found with reference to commonly-assigned U.S. Pat. No. 3,985,578 to Witherspoon et al.; U.S. Pat. No. 5,272,017 to Swathirajan et al.; U.S. Pat. No. 5,624,769 to Li et al.; U.S. Pat. No. 5,776,624 to Neutzler; U.S. Pat. No. 6,277,513 to Swathirajan et al.; U.S. Pat. No. 6,350,539 to Woods, III et al.; U.S. Pat. No. 6,372,376 to Fronk et al.; U.S. Pat. No. 6,521,381 to Vyas et al.; U.S. Pat. No. 6,524,736 to Sompalli et al.; U.S. Pat. No. 6,566,004 to Fly et al.; U.S. Pat. No. 6,663,994 to Fly et al.; U.S. Pat. No. 6,793,544 to Brady et al.; U.S. Pat. No. 6,794,068 to Rapaport et al.; U.S. Pat. No. 6,811,918 to Blunk et al.; U.S. Pat. No. 6,824,909 to Mathias et al.; U.S. Patent Application Publication Nos. 2005/0026012 to O'Hara; 2005/0026018 to O'Hara et al.; and 2005/0026523 to O'Hara et al., the entire specifications of all of which are expressly incorporated herein by reference.
The gas diffusion media component of a PEM fuel cell is typically comprised of a non-woven carbon fiber paper, e.g., those available from Toray Industries, Inc. (Tokyo, Japan), or a woven carbon cloth, e.g., those available from Zoltek Corporation (St. Louis, Mo.) under the PANEX trade name. Upon arrival at the PEM fuel cell manufacturer, the as-is product is typically post treated in order to render the material hydrophobic. Additionally, it has become more common practice to apply a microlayer ink (sometimes referred to as a microporous layer (MPL)) to the gas diffusion layer (GDL) for more effective water management properties.
A typical microlayer ink generally includes a carbon black powder and a fluorinated polymer dispersion (typically PTFE) that is contained in a solvent system. Historically, stabilizing carbon black powder. in dispersions has been a challenge to ink specialists in numerous industries, including those involved with the production of PEM fuel cells. For a PEM fuel cell application, choosing the correct carbon black and polymer for optimal performance is another complexity. As such, current approaches of forming stable microlayer ink compositions, especially for use in conjunction with high current density (HCD) applications, are extremely difficult and have not resulted in completely satisfactory results.
Accordingly, there exists a need for new and improved ink compositions for gas diffusion media components of PEM fuel cell systems, especially those that are both stable and perform satisfactorily in conjunction with HCD applications.